U.S. Pat. No. 5,179,347 which issued Jan. 12, 1993 to Alfred J. Hawkins shows a classical electrical sensor for sensing moisture in soils. This patent is incorporated herein in its entirety for its showing of a soil moisture sensor of which this invention is an improvement. Details of construction for most of the sensor of this invention can be learned from this patent. The related product has been manufactured and sold for many years by Irrometer Company, Inc., at Riverside Calif. This invention is an improvement on this already-proved product, providing an improved response to changes in soil moisture as the moisture flows both inwardly and outwardly of the sensor when the soil becomes wetter or drier as irrigation, weather conditions, and plant transportation change.
Within its effective range the sensor provides useful information regarding the wetness of the soil at the depth where the sensor is placed. Depending on the crop, the depths at which the sensor is buried may be relatively shallow for row crops, or very deep near the roots of established trees, or in between these depths for other crops.
The sensor includes a strong perforated housing to resist external physical loads and to confine its contents. It is preferably made of an electrically conductive metal to provide some screening against stray electrical currents in the soil. An internal cavity contains a porous transmission matrix, usually tightly packed silica sand. A liner to be described fits inside the housing and forms an interface between the soil and the transmission matrix while it also confines the matrix to the cavity and passes moisture from the soil into the matrix, and from the matrix into the soil.
A slightly soluble buffer tablet is placed atop the transmission matrix, and beneath an electrode chamber matrix. A pair of electrodes is placed in the electronic chamber, spaced apart from one another by electrode chamber matrix. Circuit leads extend from these electrodes for connection to measurement instruments.
The concentration of water in soil (wetness) is learned by measuring the electrical resistance of the path between the electrodes. In addition to information about actual wetness current information whether the soil is becoming wetter or drier and how rapidly is important to the grower. The more responsive the sensor is to the soil condition, both temporally and accurately, the better.
It is an object of this invention to provide an improvement to the interface between the soil and the transmission matrix. One of the functions of the interface is to retain the transmission matrix inside the housing, an obviously necessary function. This function is provided in the Hawkins patent by a liner inside the housing. The other function of the interface is optimally to transfer moisture to-or from the transmission matrix from or to the soil so the wetness of the transmission matrix will approximate or equal the wetness of the soil.
In the Hawkins patent the interface is a liner made of a woven medium. It is a filter in the sense that it retains particles of a given size and larger. This material has sufficient strength for structural purposes, and open passages through which the moisture can flow. Depending on the liner material, the liner may itself become involved in the transmission of moisture by itself soaking and drying out. This delays the transmission of moisture during the time that some of the moisture soaks the matrix and stays in it, or leaves the filter material drier. This can cause a delay in response while the interface material itself adjusts to the change in wetness just at the time of most interest.
It is an object of this invention to provide for this sensor a liner made of fibers having limited capacity for moisture, randomly laid and bonded, not woven, in a pattern with passages of various and irregular cross-sections all of which will retain the silica sand but as a group is less likely to become clogged by these particles, than an orderly weave of porous filter paper As a consequence the moisture is more readily transferred through the liner into and out of the transmission matrix. When provided, a hydrophilic surface appears to facilitate passage of the moisture through the liner. Substantial absorption or loss of moisture in the interface material itself is avoided by limiting the depth of hydrophilic layer when this layer has the property of limited absorption of the water and laying it atop a cord of material that does not absorb water.